Electron lens



H. G. LUBszYNsKI ELECTRON LENS Filed May l2 1938 June 1l, 1940.

|NVENTQR H. G. L z/sz YNSK/ B 5w ATTORNEY A' :UNITED STATES `PatentedJune 11,1940

PATENT OFFICE ELECTRON LENS Hans Gerhard Lubszynsk-i, Hillingdon,Middlesex, England, assigner to Electric & Musical Inl `dustriesLimited, Hayes, Middlesex, England, a

company of Great Britain Application May 12, 193s, serial No. 207,470 wv 11n Great Britain May 20, 1937 l,

The present invention relates to electron discharge devices comprisingelectron optical systems.` o, t

",I'heinvention is; particularly but not exc1usively concerned with theposition of. lenses for useyin electron dischargedevices in` which anoptical ,image oianrobject is `projected on Ito a photo-sensitivecathode and `the photoelectron image emanating from the cathode beingformed by a lens on to arionfphoto` sensitive mosaic elec-` trode orscreen which is arranged to be scanned bye cathode ray beam to producepicture signals, for example, in a television transmission system. Anelectron discharge device of this lrind, termed an image transmitter,"is disclosed inthe speci,n fication of British Patent No. 442,666.

There Iare'twomfactors whiohcontrol the choice of electron lenses usedin the above mentioned devices, namely `the deiinition required in theelectron `image and the distortion produced by the electron` lenses.`From theV point of view of i definition and from energy considerations,it is required that the area of the electron image and `mosaic electrodeby an electromagnetic coil, the

distortion in the electron image increases with the size of the image toan extent depending on the ratio of the coil diameter to the diameter oithe electron image. The coil diameter is restricted due to the fact thatwith a large coil the magnetic field produced would ,spread too faralong the i axis of the coil, and would tend to interfere with thescanning beam, and also with some constructions the length of the lenscoil used and/or the cross-section ci the electron beam mayl be limitedby the geometry of the envelope or tube enclosing the electrodes.` Alsoin the case of an electrostatic lens, if the crosssection of the elec,-

o tron beam is large in comparison with the separation of the plates bywhich the lens iielcl is produced, a serious amount of sphericalaberration will appeal` in the image obtained.

The object of the present invention is to provide an improved electronoptical system which may beused in electron discharge devices to focus alarge electron image with small distortion or to produce an electronimage in cases Where the `cross-section of the electron beamisrestricted in some parts of its path.

` According to the `present invention, an electron discharge device isprovided comprising a source of electrons from which a `beam ofelectrons is projected along a path, a long frusto-conioalelectro-,magnetic coil with its larger end directed towards said sourcearranged about said` path and adapted to be energized so `as toset up amagnetic lens field which causes the cross-section of the :electronbeam` projected from said source to become `reduced and the beam ,tobecome `or tend to become focused, there beinga further electron lens`constituting device arranged `to be energized `to set up a further lens`field for `acting on said electron beam after it-has .been acted on bythe rst lens field to produce an electron 15 image.

Themethod of carrying the invention intorlpractice will be readilyunderstood from the following description in detail, reference beingmade to the single figure oi the drawing which representsdiagrainmatically an arrangement according to the invention as appliedby way of example to an image transmitter of the kind referred to above.In the arrangement shown in the drawing, P is a large photo-cathode onwhich the imageoi an object O is projectedby an optical system represented by lens L. This photo-cathode P is ar ranged on the end of aconical neck portion l of the enclosing envelope 2 of a scanning tube,the electron gun `of which is shown at G. The neck portion of the tubetapers away from the photo-cathode. In body portion 3 of the envelope isarranged a mosaic electrode M which is adapted to be scanned by acathode ray, de veloped by the electron gun G. Surrounding the conicalneck portion l and extending slightly beyond the photoathode P, asshown, is a. conica-1 electromagnetic coil 4 which, as described, forexample, by Coeterier and Teves in Physica v vol. 3, 1936, pages698-976, reduces the crosssection of the electron beam emitted from the4o photo-cathode P as indicated by the dotted lines in the drawing.Surrounding the body portion 3 of the envelope is a further shortelectro-magnetic coil 5, by which the electron beam of reducedcrosssection is focused `on the mosaic electrode M, the focused electronimage being of such size as to occupy practically the whole of theavailable area on the mosaic electrode. With this arrangement thecross-section of the 50 electron beam after the beam has passed throughthe coil 4 will be small and there will only be a small amount ofdistortion present, and the image produced by the coll 4 will be foundto be both large and free from serious distortion.` Moreover, the use ofthe conical coil 4 as described enables the cross-section of the neck Iat its point of junction with the body portion 3 to be made relativelysmall. Y

While the invention described above is pri marily (intended forapplication to image transmitters, it Wil1 be appreciated that ,theinvention may also be applied to any arrangement in'which an electronimage is required to be produced on an extended surface, as for example,in an elec tron telescope or microscope where no scanning is involved. y

lIf desired, the magneticfcoil 5`might be replaced by a suitablearrangement adapted to be charged to different potentials to set upanelectrostatic iield constituting an electrostatic electron lens ratherthan an electromagnetic lens as shown in the drawing, in which casealso, the arrangement of the invention may be used to. lessen thedistortion or aberration in the image produced and to overcomediiiiculties due to the construction i of the tube.

Having now particularly described and ascertained the nature of my saidinvention and in what manner the same is to be performed, I declare thatwhat I claim is: i

l. An electron discharge device including a photo-cathode electrode uponwhich an optical image may be projected and from which an electroncurrent image may be derived, means for directing the photo-electronsconstituting the current image along a predetermined path including along `frustro-conical electromagnetic coil arranged about the path withits 'larger end directed toward said photo-cathode and adapted to beenergized so as to set up a magnetic electron lens eld which causes thecross-section of the photo-electron beam emitted by said photocathode tobecome materially reduced, means including a further electron lensdevice adapted to be energized to set up a further magnetic electronlens field for acting on said photo-electron beam, after it has beenacted on bytherst ,lens eld, to cause divergence of the beam? tojfproduce an electron image, and a mosaic electrode upon which the4produced electron image .may be focused, whereby-because of thereduction in the cross-section of he beam under the action of the iieldset up injthe vfrustro-conical coiL-fthedistortion arising is less thanthat which Would be produced by saidV `further lens device acting alone.

2. An electron discharge device including a photo-cathode electrode uponwhich an optical image may be projected and from which an electroncurrent image may be derived, means for directing the photo-electronsconstituting the current image along a predetermined path including along frustro-conical electromagnetic coil arranged concentrically aboutthe path with` its larger end directed toward said photo-cathode andadapted to be energized so as to set up a magnetic lens iield forcausing the cross-sectional area of the photo-electron beam tobecomematerially reduced, means including a second elcctromagnetic coiladapted to be energized so as to set up asecond magnetic lens eld foracting duced by reason of the convergence and divergy.

ence of the electron current image.

HANS GERHARD IlUBSZYNSKI.`

